As the population of an active society ages, medical advancements are developed to improve the health of individuals. One of the most active and successful medical treatments is joint reconstruction, also known as arthroplasty, to resolve activity limiting pain caused by arthritis. Current technology supports various forms of arthroplasty, including hemi-arthroplasty, partial joint arthroplasty and total joint arthroplasty. These successful procedures reconstruct a new continuous, low friction articular surface for pain free function of the skeletal joint. A remaining challenge in arthroplasty deals with resolving activity limiting pain in patients with smaller focal cartilage lesions. These lesions represent earlier stages of arthritis and if left untreated potentially progress to later stages of arthritis requiring a more invasive procedure such as partial or total joint replacement. The current challenge in treating arthritis lies in developing a more versatile implant for focal, regional or global resurfacing that successfully interacts with mating articular cartilage, surrounding articular cartilage and the underlying bone bed.
Diarthrodial joints in the human skeleton provide the nearly frictionless pain free movement supporting locomotion, spatial positioning relative to the environment and active manipulation of the surroundings. These skeletal joints have a strong fibrous capsule enclosing bone ends encapsulated by smooth continuous cartilage surfaces to accomplish this function. This biologic configuration represents the majority of skeletal joints in the human body.
The encapsulating surface on the ends of moving bones is known as hyaline cartilage, a hydrated soft tissue comprised of collagen, trapped proteoglycans, other proteins and chondrocytes. This tissue is more commonly known as articular cartilage or native articular cartilage. This ordered tissue provides a resilient, continuous layer of protective tissue on the bone ends. In addition to protecting the bone ends, it also helps develop an extraordinarily low coefficient of friction during joint movement, by interacting with the synovial fluid.
The resiliency of articular cartilage is supported by a dense bone layer, called the subchondral plate, which provides foundational strength for the articular cartilage. The bone side of the subchondral plate is supported by cancellous bone. Cancellous bone is a highly porous structure with a stiffness 1/10th that of the subchondral plate. The cancellous bone acts to distribute loads across the joint in the metaphyseal region of bone ends.
Skeletal joints are subject to wear and tear though use, trauma and aging. These factors eventually cause biologic changes to the articular cartilage resulting in arthritis, a group of progressive conditions ultimately resulting in irreversible damage to the articular cartilage in skeletal joints.
As damage to the affected articular cartilage surfaces progress, the smooth continuous layer of protective tissue becomes torn and discontinuous. Unlike other tissues, the body is unable to regenerate this well ordered hyaline cartilage and substitutes a less durable, rougher form of cartilage known as fibrocartilage. This less protective fibrocartilage increases the coefficient of friction in the joint and results in a greater volume of microfracturing in the cancellous bone. In reaction to this structural breakdown, the body reacts by thickening the subchondral plate to assist in distributing the load across the bone end. Researchers have sighted this stiffening of the subchondral bone as a possible mechanism for the initiation of cartilage damage. This may be why untreated cartilage lesions cause arthritis to progress and affect larger areas of articular cartilage in a joint over time, leading to activity limiting pain and decreased joint function.
Osteoarthritis (OA) or Degenerative Joint Disease (DJD) is the most common form of arthritis and presents the patient with debilitating pain during daily activities. It is the leading cause of chronic disability in the United States in the middle-aged population, but affects people of all ages. It is estimated that 21 million people have a form of arthritis in the US, accounting for 25% of visits to primary care physicians and half of all NSAID (Non-Steroidal Anti-Inflammatory Drugs) prescriptions.
OA commonly affects the joints at the hips, knees, shoulder, elbow and spine, and small joints such as those found in the hands and feet. As a result, various methods have been developed to treat and repair damaged or destroyed articular cartilage.
For smaller defects, usually identified early in the onset of arthritis during diagnostic workups, arthroscopic debridement, abrasion arthroplasty or abrasion chondralplasty procedures are conducted. The principle behind these procedures is to stimulate bleeding of the subchondral bone bed by abrading it with a burr or shaver to stimulate the fibrocartilage healing response. Although this procedure has been widely used over the past two decades, with good short term results out to three years, the resulting fibrocartilage developed in the healed area does not always support longer term low friction weight bearing function.
Another procedure referred to as “microfracture” incorporates the concept of fibrocartilage healing by removing the damaged cartilage layer and using a surgical awl to perforate the subchondral bone. This technique creates a replacement surface similar in type and outcome to the one created from the abrasion chondralplasty technique.
Transplantation of previously harvested hyaline cartilage cells, known as cell-based therapy, has been utilized in recent years. This technique uses autologous chondrocytes obtained from a specimen of articular cartilage obtained from an uninvolved area of the injured joint. The cartilage cells are isolated, cultured and implanted in the defect area under a periosteal flap. Compared to the previously discussed abrasion techniques, this procedure requires a lengthy post-operative non-weight bearing course and is still viewed somewhat as experimental because of the technical challenges involved in the procedure producing variations in patient outcomes.
Cartilage transplant, referred to as Mosaicplasty or Osteoarticular Transfer System (OATS) is a technique utilizing articular tissue grafts in the form of plugs. These plugs consist of articular cartilage, subchondral bone and cancellous bone to assure they heal to the bone and surrounding articular cartilage in the surgically prepared defect region.
Two different types of donor plugs are harvested for this procedure. The first is taken from a matched articular location in a cadaver bone (allograft). The second type is taken directly from the patient (autograft) in boundary or non-weight bearing locations in the joint being reconstructed.
In either case, the technique for utilizing articular cartilage grafts is challenging. Success of the technique requires accurate harvesting and positioning of single or multiple plugs to reconstruct the articular surface of the subject joint. The plug must be harvested perpendicular to the articular surface, then positioned perpendicular and flush with the retained articular cartilage surrounding the defect area. If the grafts are placed too far below the level of the surrounding articular surface, no benefit from the procedure will be gained and cartilage damage can progress beyond the perimeter of the original defect. If the grafts are placed proud to the surrounding articular surface, detrimental effects can be seen on the mating articular surface over time in the joint. This is important to consider since arthritis often affects one side of an articular joint first before progressing to the mating surface.
The result of positioning these plugs in a mosaic-like fashion establishes a new hyaline cartilage surface. The result is a hyaline-like surface interposed with a fibrocartilage healing response between each graft. In addition to the many challenges discussed surrounding this procedure, a lengthy post-operative non-weight bearing course is required to improve the patient's chance for success in restoring functional articular cartilage in the skeletal joint.
Other clinical challenges exist beyond the technique issues previously discussed. In the case of allograft plugs graft availability, potential disease transmission and tissue quality are all concerns. In the case of autograft plugs, the quantity and articular shape of available tissue create limitations in the defect size to be treated.
Advances in tissue engineering are beginning to provide treatments to repair focal cartilage lesions in skeletal joints by implanting collagen based scaffold devices, with and without impregnated autologous chondrocytes (cartilage cells). This reconstructive technique, referred to as scaffold guided regeneration, establishes a generic tissue foundation which is converted over time by the body into hyaline cartilage. Initial results using this reconstructive technique show promise, but are currently used in non-weight bearing applications which limit their use in reconstructive procedures presently favoring more traditional devices made from implantable metals, ultra high molecular weight polyethylene (UHMWPE) and ceramics.
One type of joint replacement technique using more traditional devices is called hemi-arthroplasty. This reconstructive procedure replaces one bone end of the two or more bone ends comprising a skeletal joint. The procedure leaves the healthy part or parts of the joint unaltered. The challenge is for the artificial implant to articulate with the native cartilage surfaces over time without recreating painful arthritis as the healthy cartilage tissue becomes arthritic. Clinical experience in using hemi-arthroplasty implants with metal articular surfaces in younger more active patients has shown undesirable thinning and damage of the mating native articular cartilage in early term follow-up. For this reason, this class of procedure is most commonly performed in older patients following a hip fracture. During hemi-arthroplasty of the hip, the surgeon removes the damaged bone and cartilage from the hip joint, usually the femoral head. The healthy mating surface in the acetabulum or pelvis is left intact. One such implant in accordance with the prior art is shown in FIG. 1A and is further described in U.S. Pat. No. 6,096,084 to Townley. The implant 20 can be used for hemi-arthroplasty or in total arthroplasty. The implant 20 may have a ceramic head 22 and a metal stem 24, which is implanted in the proximal region of the femur. The metal stem 24 in Townley is made of cobalt chrome, which is a cobalt-chromium-molybdenum alloy, a metal alloy often used for reconstructive implants. The stem provides a means for fixing the implant to bone to stabilize the artificial articular surface. Similar devices to this hip implant are used in the shoulder, knee, ankle, hands and feet.
When arthritis progresses to all aspects of an articular joint a total joint arthroplasty is performed to reconstruct the cartilage on all bone ends making up the skeletal joint. This comprehensive procedure is required to effectively resolve the activity limiting pain caused by the arthritis. In a total knee, for example, a highly polished metal implant is placed onto the distal femur. A modular metal tray is implanted in the proximal tibia and a UHMWPE bearing joined to it to articulate with the highly polished femoral component. A UHMWPE patellar implant is placed to resurface the patella and articulate against the anterior flange of highly polished femoral implant. This completely resurfaces the femoral-tibial and patella-femoral articular surfaces in the total knee replacement.
The risks involved in joint arthroplasty described previously include mal-position of the components, skeletal loosening, instability/dislocation, loss of range of motion and recurring activity limiting pain.
One long term risk is loosening of the components, because the bond between the bone and the components or the cement may break down or fatigue. Various approaches in the prior art attempt to address the loosening risk. For example, U.S. Pat. No. 6,685,987 describes a porous coating comprised of metallic particles applied over a cobalt chromium molybdenum alloy implant.
Generally joint replacement bearing surfaces are made of cobalt chromium; however other materials have been used or proposed including titanium and titanium alloys. U.S. Patent Application Publication No. 2005/0107888 to Khandkar et al. describes a metal-ceramic composite for joint replacement materials. U.S. Pat. No. 6,398,815 to Pope et al. describes a prosthetic joint with diamond like surfaces.
As described above, the replacement with prosthetic joints is currently the preferred option for serious degeneration of joint function involving loss of articular cartilage. Other techniques include U.S. Pat. No. 7,029,479 to Tallarida et al. that discloses a method for joint resurface repair which involves mapping and measuring the articular surface, U.S. Pat. No. 5,782,835 to Hart et al. that discloses an apparatus and method for repair of articular cartilage including a bone plug removal tool, and a bone plug emplacement tool, U.S. Pat. No. 6,679,917 to Ek that discloses an implant for installation into a portion of an articular surface including a protrusion configured to cover an un-excised portion of the articular surface proximate to the implant, U.S. Pat. No. 5,413,608 to Keller that discloses a knee joint endoprosthesis for replacing the articular surfaces of the tibia comprising a bearing part that is anchored on the bone having an upper bearing surface and a rotatable plateau secured on the bearing surface and forming a part of the articular surface to be replaced, U.S. Pat. No. 5,632,745 to Schwartz that describes a method of surgically implanting into a site a bio-absorbable cartilage repair assembly, U.S. Pat. No. 5,683,466 to Vitale that discloses an articular joint surface replacement system having two opposing components, U.S. Pat. No. 5,702,401 to Shaffer that discloses an intra-articular measuring device including a hollow handle defining a first passageway and a hollow tube having a second passageway extending from the handle, and U.S. Pat. No. 5,771,310 to Vannah that describes a method of mapping the three-dimensional topography of the surface of an object by generating digital data points at a plurality of sample points on said surface. Another implant is described in U.S. Publication No. 2003/0074081 to Ayers that describes a method for production of tissue implants and prosthetics. U.S. Publication No. 2007/0113951 to Huang describes an osteochondral composite scaffold for articular cartilage repair.
Another orthopedic procedure involves fusing bones together and is clearly distinct from joint replacement. One such application is for spinal fusion. For example U.S. Patent Application Publication No. 2005/0049706 and U.S. Pat. No. 6,790,233 to Brodke et al. describe radio lucent spinal fusion cages, one of which is shown in FIG. 1B. The cage includes a substrate block 30 having a high bio-mechanical strength and load bearing capacity to support the spinal vertebrae 32 and a porous silicon nitride ceramic portion 34 to promote bone ingrowth and fusion. Other examples of fusing bones together include U.S. Patent Application Publication No. 2006/0271201 to Kumar et al. that describes using porous ceramic 36 to repair defects in bone 38, as shown in FIG. 1C, and U.S. Pat. No. 6,607,557. Because these devices are intended to fuse bones together, they are inappropriate for repair of damaged joints which by their nature should have free movement.
The reconstructive prior art methods for articular cartilage repair previously discussed have disadvantages and drawbacks related to treating early stage arthritis to prevent progression to a more final stage requiring total joint replacement.
What is needed is a more versatile articular orthopedic implant to function in a collaborative environment with native tissue. Also needed is a non-resorbable implant to support loads imposed by an opposing joint end. In particular what is needed is an implant that will facilitate surgical repair of focal, regional and global articular cartilage and osteochondral defects on a bone end of a skeletal joint to prevent or delay the global progression of arthritis to the entire joint. The embodiments of the present disclosure answer these and other needs.